1. Field Of The Invention
The invention relates to recovering polyoxyalkylene polymers from aqueous liquids. The invention is also a method of treating wastewater.
2. Description Of The Related Art
Polyoxyalkylene glycols are dissolved in water to produce aqueous fluids which demonstrate a number of advantageous physical and chemical properties. For many applications, the water solubility and lubricity of polyoxyalkylene glycols are the properties which determine the use of the fluid. In combination with water, the property described as inverse solubility enhances the lubricating property of the fluid. Inverse solubility refers to the property of certain polyoxyalkylene glycols to be soluble in water at lower temperatures and become insoluble, cloudy or unstable at some elevated temperature. This cloudiness or instability indicates phase separation from water which results in polyoxyalkylene glycol deposition on surfaces in contact with the fluid. The temperature at which instability and insolubility occurs is called the cloud point of the polyoxyalkylene glycol.
Friction in a machine results in the heating of lubricated surfaces. If the temperature of heated surfaces exceeds the cloud point, polyoxyalkylene glycol deposits on the surfaces, providing extra lubricity, reducing friction and wear. This deposition is desirable and is the mechanism by which some polyoxyalkylene glycols enhance lubricity.
Polyoxyalkylene glycols comprise epoxide monomers, most typically ethylene oxide, propylene oxide, butylene oxide and mixtures thereof. The polymerization is initiated with a compound having an active hydrogen such as an alcohol, glycol, glycerine, thio-alcohol, or sugar. The initiator influences a number of the physical and chemical properties of the glycol. The physical properties of a polyoxyalkylene glycol are also a function of the monomer as well as, proportion and distribution of monomers and the molecular weight.
Not all polyoxyalkylene glycols are water-soluble, and not all those that are water-soluble exhibit a cloud point. Those which do exhibit a combination of lubricity, water solubility, and a cloud point are generally alcohol initiated and composed of a random mix of approximately equal amounts of ethylene oxide (EO) and propylene oxide (PO) along the polymer chain. They have cloud points between 40.degree. C. and 70.degree. C., and a molecular weight of 750 or greater. These polyoxyalkylene glycols exhibit the preferred combination of water solubility, lubricity, and inverse solubility. Some EO/PO block copolymers also have cloud points between 30.degree. C. and 80.degree. C., but they are more commonly used as surfactants rather than as functional fluid.
After a polyoxyalkylene glycol fluid is used for a period of time, it becomes reduced in effectiveness and contaminated. If the fluid is disposed of, disposal typically includes some method of separating the polyoxyalkylene glycol from water. Methods used to separate polyoxyalkylene glycol from water include chemical treatment, mechanical separation and filtration. Each unit operation requires handling and significant cost due to treatment chemicals, equipment, and heating.
It is known that polyoxyalkylene glycol can be separated from a spent aqueous fluid by heating the entire fluid to a temperature above the cloud point. The polyoxyalkylene glycol separates from the water and settles to the bottom of the fluid. There it is mechanically separated from the water. This method requires the energy to heat the entire fluid and the equipment to contain the quiescent fluid during precipitation.
There is a need in the art for an energy efficient method of recovering polyoxyalkylene glycols from water.